This invention relates to the fabrication of integrated circuits and, more particularly, to techniques for precisely aligning mask and wafer members utilized in making such circuits.
The fabrication of microminiature devices and circuits often requires that each of a set of masks (sometimes as many as 10 to 12) be successively aligned with respect to a semiconductor wafer. To obtain reasonable yields in the manufacture of such devices, precise tolerances are required in the alignment process. For very-high-resolution devices, sub-micron alignment tolerances are often necessary. In practice, the problem of devising satisfactory instrumentalities for achieving such highly precise alignment has been a difficult one to solve.
X-ray lithography is being increasingly recognized as an attractive approach for realizing very-high-resolution devices. But one of the several major problems faced by designers of X-ray exposure systems has been the requirement that the mask-to-wafer alignment capability thereof have a sub-micron precision. Moreover, if an optically based alignment technique is desired, designers are faced with the additional problem that the mask overlying an associated wafer in such a system is typically not highly transmissive to the light used for alignment. As a result, the brightness and contrast of the marks observed on the wafer during the alignment process are sometimes marginal in character. Furthermore, known alignment techniques are not capable of compensating for so-called magnification errors that arise in an X-ray system due to distortions in the mask and/or wafer, or due to other causes that result in mask-to-wafer spacing variations.
Accordingly, considerable effort has been directed at trying to devise an improved optical alignment technique for use in particular in an X-ray lithographic system. It was recognized that such effort, if successful, would significantly enhance the capability of the system and increase the likelihood of widespread commercial utilization thereof for fabricating very-high-resolution devices.